U.S. patent application number 17/658049 was filed with the patent office on 2022-07-21 for systems and methods for production of silicon using a horizontal magnetic field.
The applicant listed for this patent is GlobalWafers Co., Ltd.. Invention is credited to Carissima Marie Hudson, JunHwan Ji, Richard J. Phillips, JaeWoo Ryu, WooJin Yoon.
Application Number | 20220228291 17/658049 |
Document ID | / |
Family ID | 1000006242360 |
Filed Date | 2022-07-21 |
United States Patent
Application |
20220228291 |
Kind Code |
A1 |
Ryu; JaeWoo ; et
al. |
July 21, 2022 |
SYSTEMS AND METHODS FOR PRODUCTION OF SILICON USING A HORIZONTAL
MAGNETIC FIELD
Abstract
A method for producing a silicon ingot by the horizontal
magnetic field Czochralski method includes rotating a crucible
containing a silicon melt, applying a horizontal magnetic field to
the crucible, contacting the silicon melt with a seed crystal, and
withdrawing the seed crystal from the silicon melt while rotating
the crucible to form a silicon ingot. The crucible has a wettable
surface with a cristobalite layer formed thereon.
Inventors: |
Ryu; JaeWoo; (Chesterfield,
MO) ; Ji; JunHwan; (Cheonan-si, KR) ; Yoon;
WooJin; (Cheonan-si, KR) ; Phillips; Richard J.;
(St. Peters, MO) ; Hudson; Carissima Marie; (St.
Charles, MO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GlobalWafers Co., Ltd. |
Hsinchu |
|
TW |
|
|
Family ID: |
1000006242360 |
Appl. No.: |
17/658049 |
Filed: |
April 5, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
17115154 |
Dec 8, 2020 |
|
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|
17658049 |
|
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62947785 |
Dec 13, 2019 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 15/10 20130101;
C30B 30/04 20130101; C30B 15/30 20130101; C30B 15/20 20130101; C30B
29/06 20130101 |
International
Class: |
C30B 15/10 20060101
C30B015/10; C30B 15/20 20060101 C30B015/20; C30B 15/30 20060101
C30B015/30; C30B 29/06 20060101 C30B029/06; C30B 30/04 20060101
C30B030/04 |
Claims
1. A system for producing a silicon ingot, the system comprising: a
crucible to contain a silicon melt, the crucible having a wettable
surface with a cristobalite layer formed thereon; magnetic poles to
produce a horizontal magnetic field; a controller programmed to
produce a silicon ingot by: rotating the crucible containing the
silicon melt; applying a horizontal magnetic field to the crucible
using the magnetic poles; contacting the silicon melt with a seed
crystal; and withdrawing the seed crystal from the silicon melt
while rotating the crucible to form a silicon ingot.
2. The system of claim 1, wherein the crucible comprises a natural
sand crucible.
3. The system of claim 2, wherein the controller is further
programmed to add a melt modifier to the natural sand crucible to
form the cristobalite layer on the wettable surface of the natural
sand crucible.
4. The system of claim 3, wherein the controller is programmed to
add the melt modifier while heating polycrystalline silicon in the
natural sand crucible to form the silicon melt.
5. The system of claim 3, wherein the controller is programmed to
add the melt modifier after the silicon melt has been formed.
6. The system of claim 3, wherein the melt modifier comprises
barium carbonate (BaCO.sub.3).
7. The system of claim 6, wherein the controller is programmed to
add more than 1.7 grams of barium carbonate per square meter of the
wettable surface of the natural sand crucible.
8. The system of claim 3, wherein the controller is programmed to
add one of barium oxide (BaO) or strontium carbonate (SrCO.sub.3)
as the melt modifier.
9. The system of claim 8, wherein the controller is programmed to
add one of barium oxide or strontium carbonate in an amount
substantially equivalent to 1.7 grams of barium carbonate per
square meter of the wettable surface of the natural sand
crucible.
10. The system of claim 1, wherein the controller is programmed to
rotate the crucible at more than two revolutions per minute.
11. The system of claim 1, wherein the cristobalite layer is
greater than about 2.00 mm thick.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 17/115,154 filed Dec. 8, 2020, which claim priority to
U.S. Provisional Patent Application No. 62/947,785 filed Dec. 13,
2019, the entire disclosures of which are hereby incorporated by
reference in their entireties.
FIELD
[0002] This disclosure generally relates to the production of
silicon ingots, and more specifically, to methods and systems for
achieving a high success ratio producing silicon ingots in the
Czochralski process using a horizontal magnetic field.
BACKGROUND
[0003] During the 1990's, at least some high quality silicon growth
was mainly controlled by the thermal condition of the puller and
more specifically the hot zone (HZ) design itself, because the
ratio of the pulling speed to the thermal gradient (v/G) was
considered the dominant factor. In the late 1990's, further
consideration of the crystal/melt interface at the same v/G was
included in the growth of at least some high quality silicon. At
that time, application of high quality silicon to memory devices
really expanded as more customers transitioned from epi to polished
and from 200 mm to 300 mm silicon. Soon after, it became
established that high quality silicon growth requires very stable
process growth conditions and controlled melt flow to achieve the
specific crystal/melt needed to achieve the desired low crystal
defectivity during growth.
[0004] Peripherally, as silicon crystal growth transitioned from
200 mm to 300 mm and corresponding charge sizes increased to
maintain productivity, the need for magnetic field application to
stabilize melt flow in the increasing melt volume was recognized as
a dominate feature.
[0005] Several silicon manufactures transitioned to a horizontal
magnetic field Czochralski process (HMCZ) in the early 2000's when
high quality 300 mm silicon production started in order to control
the crystal/melt interface effectively. Other silicon manufactures
used a cusp magnetic field for 300 mm production of high quality
silicon. In both cases, magnetic field in the silicon melt had a
dramatic impact on crystal quality and performance and every
manufacturer developed their own technique to optimize performance
and quality from the onset.
[0006] During the process of producing single crystal silicon
ingots with the CZ process and a magnetic field, oxygen may be
introduced into silicon crystal ingots through a melt-solid or melt
crystal interface. The oxygen may cause various defects in wafers
produced from the ingots, reducing the yield of semiconductor
devices fabricated using the ingots. For example, memory devices,
insulated-gate bipolar transistors (IGBTs), high quality
radio-frequency (RF), high resistivity silicon on insulator
(HR-SOI), and charge trap layer SOI (CTL-SOI) applications
typically require a low interstitial oxygen concentration (Oi) in
order to achieve high resistivity. In the case of HMCZ process, it
was believed that the process typically requires a very low
crucible rotation (C/R) to control oxygen in the growing crystal,
particularly to control oxygen inclusion to the desired range
applicable for memory devices. Further, a higher occurrence ratio
of lost zero dislocation (LZD) from the crown to the end of body
was found in HMCZ as compared to processes using a cusp magnetic
field.
[0007] Thus, there exists a need for methods and systems that
reduce LZD losses with HMCZ growth and provide an improved ZD
success ratio for high quality silicon growth from crown to
body.
[0008] This background section is intended to introduce the reader
to various aspects of art that may be related to various aspects of
the present disclosure, which are described and/or claimed below.
This discussion is believed to be helpful in providing the reader
with background information to facilitate a better understanding of
the various aspects of the present disclosure. Accordingly, it
should be understood that these statements are to be read in this
light, and not as admissions of prior art.
BRIEF SUMMARY
[0009] In one aspect of this disclosure, a method for producing a
silicon ingot by the horizontal magnetic field Czochralski method
includes rotating a crucible containing a silicon melt, applying a
horizontal magnetic field to the crucible, contacting the silicon
melt with a seed crystal, and withdrawing the seed crystal from the
silicon melt while rotating the crucible to form a silicon ingot.
The crucible has a wettable surface with a cristobalite layer
formed thereon
[0010] Another aspect is a wafer generated from a silicon ingot
produced using the method described above.
[0011] Another aspect is a system for producing a silicon ingot.
The system includes a crucible to contain a silicon melt, magnetic
poles to produce a horizontal magnetic field, and a controller. The
crucible has a wettable surface with a cristobalite layer formed
thereon. The controller is programmed to produce a silicon ingot by
rotating the crucible containing the silicon melt, applying a
horizontal magnetic field to the crucible using the magnetic poles,
contacting the silicon melt with a seed crystal, and withdrawing
the seed crystal from the silicon melt while rotating the crucible
to form a silicon ingot.
[0012] Various refinements exist of the features noted in relation
to the above-mentioned aspect. Further features may also be
incorporated in the above-mentioned aspect as well. These
refinements and additional features may exist individually or in
any combination. For instance, various features discussed below in
relation to any of the illustrated embodiments may be incorporated
into the above-described aspect, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0014] FIG. 1 is a top view of a crucible of one embodiment.
[0015] FIG. 2 is a side view of the crucible shown in FIG. 1.
[0016] FIG. 3 is a schematic illustrating a horizontal magnetic
field applied to a crucible containing a melt in a crystal growing
apparatus.
[0017] FIG. 4 is a block diagram of a crystal growing system.
[0018] FIG. 5 presents temperature field in melt free surface by
MGP in mm and crucible rotation in RPM.
[0019] FIG. 6 is a graph of crystallization rate as a function of
time.
[0020] FIG. 7 is a graph of the thickness of the layer formed as a
function of time at a temperature of 1360.degree. C.
[0021] FIG. 8 is a contour plot of ZD success ratio by crucible
rotation and the quantity of melt modifier in a natural sand
crucible.
[0022] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0023] Referring initially to FIGS. 1 and 2, a crucible of one
embodiment is indicated generally at 10. A cylindrical coordinate
system for crucible 10 includes a radial direction R 12, an angular
direction .theta. 14, and an axial direction Z 16. The crucible 10
contains a melt 25 having a melt surface 36. A crystal 27 is grown
from the melt 25. The melt 25 may contain one or more convective
flow cells 17, 18 induced by heating of the crucible 10 and
rotation of the crucible 10 and/or crystal 27 in the angular
direction .theta. 14. The structure and interaction of these one or
more convective flow cells 17, 18 are modulated via regulation of
one of more process parameters and/or the application of a magnetic
field as described in detail herein below.
[0024] FIG. 3 is a diagram illustrating a horizontal magnetic field
being applied to crucible 10 containing melt 25 in a crystal
growing apparatus. As shown, crucible 10 contains silicon melt 25
from which a crystal 27 is grown. The transition between the melt
and the crystal is generally referred to as the crystal-melt
interface (alternatively the melt-crystal, solid-melt or melt-solid
interface) and is typically non-linear, for example concave, convex
or gull-winged relative to the melt surface. Two magnetic poles 29
are placed in opposition to generate a magnetic field generally
perpendicular to the crystal-growth direction and generally
parallel to the melt surface 36. The magnetic poles 29 may be a
conventional electromagnet, a superconductor electromagnet, or any
other suitable magnet for producing a horizontal magnetic field of
the desired strength. Application of a horizontal magnetic field
gives rise to Lorentz force along axial direction, in a direction
opposite of fluid motion, opposing forces driving melt convection.
The convection in the melt is thus suppressed, and the axial
temperature gradient in the crystal near the interface increases.
The melt-crystal interface then moves upward to the crystal side to
accommodate the increased axial temperature gradient in the crystal
near the interface and the contribution from the melt convection in
the crucible decreases. The horizontal configuration has the
advantage of efficiency in damping a convective flow at the melt
surface 36.
[0025] FIG. 4 is a block diagram of a crystal growing system 100.
System 100 employs a Czochralski crystal growth method to produce a
semiconductor ingot. In this embodiment, system 100 is configured
to produce a cylindrical semiconductor ingot having an ingot
diameter of one-hundred and fifty millimeters (150 mm), greater
than one-hundred fifty millimeters (150 mm), more specifically in a
range from approximately 150 mm to 460 mm, and even more
specifically, a diameter of approximately three-hundred millimeters
(300 mm). In other embodiments, system 100 is configured to produce
a semiconductor ingot having a two-hundred millimeter (200 mm)
ingot diameter or a four-hundred and fifty millimeter (450 mm)
ingot diameter. In addition, in one embodiment, system 100 is
configured to produce a semiconductor ingot with a total ingot
length of at least nine hundred millimeters (900 mm). In some
embodiments, the system is configured to produce a semiconductor
ingot with a length of one thousand nine hundred and fifty
millimeters (1950 mm), two thousand two hundred and fifty
millimeters (2250 mm), two thousand three hundred and fifty
millimeters (2350 mm), or longer than 2350 mm. In other
embodiments, system 100 is configured to produce a semiconductor
ingot with a total ingot length ranging from approximately nine
hundred millimeters (900 mm) to twelve hundred millimeters (1200
mm), between approximately 900 mm and approximately two thousand
millimeters (2000 mm), or between approximately 900 mm and
approximately two thousand five hundred millimeters (2500 mm). In
some embodiments, the system is configured to produce a
semiconductor ingot with a total ingot length greater than 2000
mm.
[0026] The crystal growing system 100 includes a vacuum chamber 101
enclosing crucible 10. A side heater 105, for example, a resistance
heater, surrounds crucible 10. A bottom heater 106, for example, a
resistance heater, is positioned below crucible 10. During heating
and crystal pulling, a crucible drive unit 107 (e.g., a motor)
rotates crucible 10, for example, in the clockwise direction as
indicated by the arrow 108. Crucible drive unit 107 may also raise
and/or lower crucible 10 as desired during the growth process.
Within crucible 10 is silicon melt 25 having a melt level or melt
surface 36. In operation, system 100 pulls a single crystal 27,
starting with a seed crystal 115 attached to a pull shaft or cable
117, from melt 25. One end of pull shaft or cable 117 is connected
by way of a pulley (not shown) to a drum (not shown), or any other
suitable type of lifting mechanism, for example, a shaft, and the
other end is connected to a chuck (not shown) that holds seed
crystal 115 and crystal 27 grown from seed crystal 115.
[0027] Crucible 10 and single crystal 27 have a common axis of
symmetry 38. Crucible drive unit 107 can raise crucible 10 along
axis 38 as the melt 25 is depleted to maintain melt level 36 at a
desired height. A crystal drive unit 121 similarly rotates pull
shaft or cable 117 in a direction 110 opposite the direction in
which crucible drive unit 107 rotates crucible 10 (e.g.,
counter-rotation). In embodiments using iso-rotation, crystal drive
unit 121 may rotate pull shaft or cable 117 in the same direction
in which crucible drive unit 107 rotates crucible 10 (e.g., in the
clockwise direction). Iso-rotation may also be referred to as a
co-rotation. In addition, crystal drive unit 121 raises and lowers
crystal 27 relative to melt level 36 as desired during the growth
process.
[0028] According to the Czochralski single crystal growth process,
a quantity of polycrystalline silicon, or polysilicon, is charged
to crucible 10. A heater power supply 123 energizes resistance
heaters 105 and 106, and insulation 125 lines the inner wall of
vacuum chamber 101. A gas supply 127 (e.g., a bottle) feeds argon
gas to vacuum chamber 101 via a gas flow controller 129 as a vacuum
pump 131 removes gas from vacuum chamber 101. An outer chamber 133,
which is fed with cooling water from a reservoir 135, surrounds
vacuum chamber 101.
[0029] The cooling water is then drained to a cooling water return
manifold 137. Typically, a temperature sensor such as a photocell
139 (or pyrometer) measures the temperature of melt 25 at its
surface, and a diameter transducer 141 measures a diameter of
single crystal 27. In this embodiment, system 100 does not include
an upper heater. The presence of an upper heater, or lack of an
upper heater, alters cooling characteristics of crystal 27.
[0030] Magnetic poles 29 are positioned outside the vacuum chamber
101 to produce a horizontal magnetic field (shown in FIG. 3).
Although illustrated approximately centered on the melt surface 36,
the position of the magnetic poles 29 relative to the melt surface
36 may be varied to adjust the position of the maximum gauss plane
(MGP) relative to the melt surface 36. A reservoir 153 provides
cooling water to the magnetic poles 29 before draining via cooling
water return manifold 137. A ferrous shield 155 surrounds magnetic
poles 29 to reduce stray magnetic fields and to enhance the
strength of the field produced.
[0031] A control unit 143 is used to regulate the plurality of
process parameters including, but not limited to, at least one of
crystal rotation rate, crucible rotation rate, and magnetic field
strength. In various embodiments, the control unit 143 may include
a memory 173 and processor 144 that processes the signals received
from various sensors of the system 100 including, but not limited
to, photocell 139 and diameter transducer 141, as well as to
control one or more devices of system 100 including, but not
limited to: crucible drive unit 107, crystal drive unit 121, heater
power supply 123, vacuum pump 131, gas flow controller 129 (e.g.,
an argon flow controller), magnetic poles power supply 149, and any
combination thereof. The memory 173 may store instructions that,
when executed by the processor 144 cause the processor to perform
one or more of the methods described herein. That is, the
instructions configure the control unit 143 to perform one or more
methods, processes, procedures, and the like described herein.
[0032] Control unit 143 may be a computer system. Computer systems,
as described herein, refer to any known computing device and
computer system. As described herein, all such computer systems
include a processor and a memory. However, any processor in a
computer system referred to herein may also refer to one or more
processors wherein the processor may be in one computing device or
a plurality of computing devices acting in parallel. Additionally,
any memory in a computer device referred to herein may also refer
to one or more memories wherein the memories may be in one
computing device or a plurality of computing devices acting in
parallel. Further, the computer system may located near the system
100 (e.g., in the same room, or in an adjacent room), or may be
remotely located and coupled to the rest of the system via a
network, such as an Ethernet, the Internet, or the like.
[0033] The term processor, as used herein, refers to central
processing units, microprocessors, microcontrollers, reduced
instruction set circuits (RISC), application specific integrated
circuits (ASIC), logic circuits, and any other circuit or processor
capable of executing the functions described herein. The above are
examples only, and are thus not intended to limit in any way the
definition and/or meaning of the term "processor." The memory may
include, but is not limited to, random access memory (RAM) such as
dynamic RAM (DRAM) or static RAM (SRAM), read-only memory (ROM),
erasable programmable read-only memory (EPROM), electrically
erasable programmable read-only memory (EEPROM), and non-volatile
RAM (NVRAM).
[0034] In one embodiment, a computer program is provided to enable
control unit 143, and this program is embodied on a computer
readable medium. The computer readable medium may include the
memory 173 of the control unit 143. In an example embodiment, the
computer system is executed on a single computer system.
Alternatively, the computer system may comprise multiple computer
systems, connection to a server computer, a cloud computing
environment, or the like. In some embodiments, the computer system
includes multiple components distributed among a plurality of
computing devices. One or more components may be in the form of
computer-executable instructions embodied in a computer-readable
medium.
[0035] The computer systems and processes are not limited to the
specific embodiments described herein. In addition, components of
each computer system and each process can be practiced independent
and separate from other components and processes described herein.
Each component and process also can be used in combination with
other assembly packages and processes.
[0036] In one embodiment, the computer system may be configured to
receive measurements from one or more sensors including, but not
limited to: temperature sensor 139, diameter transducer 141, and
any combination thereof, as well as to control one or more devices
of system 100 including, but not limited to: crucible drive unit
107, crystal drive unit 121, heater power supply 123, vacuum pump
131, gas flow controller 129 (e.g., an argon flow controller),
magnetic poles power supply 149, and any combination thereof as
described herein and illustrated in FIG. 4 in one embodiment. The
computer system performs all of the steps used to control one or
more devices of system 100 as described herein.
[0037] The loss of zero dislocation (ZD) structure (quantified by
LZD rate) is generally higher during silicon crystal Cz growth in a
horizontal magnet (HMCZ) field versus growth in a Cusp (or
vertical) magnetic field. However, the LZD rate in HMCZ can be
lowered dramatically if the crystal is grown in a synthetic lined
crucible versus a natural sand lined crucible. But while the ZD
rate is better, the cost of synthetic lined crucibles is higher
than natural sand. Additionally, the thin (roughly 2 mm thickness)
synthetic liner dissolves in a relatively short process time,
leaving the backing sand layer exposed to the melt, which allows
quartz particles to enter the melt and hit the growing crystal.
Thus, the exposure of bubbles in the liner or even from the backing
sand into the melt is higher. To avoid bubble exposure and/or
backing sand contamination by dissolution when using synthetic
lined crucibles, the process hot hours are generally limited to
less than .about.250 hours, which is much shorter process time than
achievable for natural sand crucibles (approximately 400-500 hours
or more). Because the hot time for crystal growth depends on the
process conditions, HZ configuration, and attempts, recharge
capability and multiple rod growth per batch can be impacted using
synthetic lined crucibles. Therefore, silicon growth using HMCZ in
synthetic lined crucibles generally requires optimization of the
crucible condition to ensure the best ZD rate and lowest attempts
so that maximum recharge capability can be achieved.
[0038] Further, a horizontal magnet in Cz growth enhances the melt
flow with an irregular velocity melt wave continuously knocking
against the crucible wall surface with a strong force in a
transient behavior. In this case, the crucible surface condition is
very critical for quartz piece generation which directly relates to
the ZD success. This is illustrated in FIG. 5, in which the melt
flow and temperature field of the melt are significantly impacted
by the magnetic field strength and MGP position. As shown in the
figure, the temperature variation of the circumferential direction
at the melt surface is markedly changed by the magnetic field
direction.
[0039] These and other difficulties may be overcome or mitigated in
embodiments of the present disclosure through use of one or both of
two techniques described in detail below. Generally, in the first
aspect, a cristobalite layer is formed on the wettable surface of
the interior of the crucible. The wettable surface generally refers
to the surface of the crucible that may be in contact with the melt
during silicon production. The wettable surface generally includes
the interior bottom of the crucible, at least a portion of the
interior sidewalls of the crucible, and the interior portions
connecting the interior sidewalls and bottom of the crucible. In
FIG. 2, the wettable surface is all of the interior surface of the
crucible 10 below and including the melt surface 36. The wettable
surface may also extend above the melt surface 36. The second
technique described in this disclosure is to increase the
rotational speed of the crucible.
[0040] In general, a strong and unsteady melt flow is induced by an
HMCZ magnetic field and this can generate strong thermo-mechanical
stress and mechanical impact on the crucible wall causing quartz
particle generation. However, high crucible rotation (C/R) will
produce faster convective flow near the crucible wall surface,
which can interfere with the melt flow driven by a magnetic field.
This will reduce the stress and impact on the wall surface.
Consequently, the generation of quartz particles at the wall
surface is reduced which in turn reduces LZDs during crystal
growth.
[0041] LZDs may also be reduced by enabling the formation of a
generally uniform crystalline SiO2 layer (referred to as
cristobalite layer) on the crucible's wettable surface. This layer
is more stable and stronger than amorphous quartz itself, and
thereby it is more resistant to melt attack by stress or mechanical
impact. Thus, the cristobalite layer reduces quartz particle
generation.
[0042] There are at least two methods to promote crystalline layer
growth on quartz crucibles. The first method is to use pre-coated
crucibles precoated with a compound, such as BaOH, that will
promote cristobalite growth, and the other is to add a suitable
melt modifier (MM) into the melt before crystal growth. Nonlimiting
examples of suitable MMs include barium (Ba) and strontium (Sr).
More specifically, nonlimiting example of suitable MMs include
barium carbonate (BaCO3), barium oxide (BaO), and strontium
carbonate (SrCO3).
[0043] Cristobalite formation on the amorphous quartz inner wall is
governed by pressure, Oi concentration, H2O and hydrogen content,
temperature, and the like. As shown in FIGS. 6 and 7, the formation
and growth of a crystalline layer on the crucible wall is governed
by the temperature of the crucible wall and concentration of the
MM, which is consumed by the crucible. FIG. 6 compares the
crystallization rate as a function of time for no MM and an 8%
AL2O3 MM. FIG. 7 compares the thickness of the layer formed as a
function of time at a temperature of 1360.degree. C. for no MM, an
8% AL2O3 MM, and a barium based MM. These charts indicates that a
proper MM addition (e.g., a barium based MM) into the melt or a
pre-coating of a barium compound generates a uniform and thick
crystalline (i.e., cristobalite) layer on the wall of natural sand
crucibles giving similar behavior and performance to synthetic
lined crucibles. Because the cristobalite layer has slower
dissolution rate than the fused quartz, the generation of quartz
pieces by thermo or mechanical stress is decreased so the crystal
LZD occurrence is decreased thereafter. Further, the generation of
secondary bubbles in the Bubble Free Layer (BFL) of the natural
sand crucible and its propagation into the melt is much less than
that of synthetic lined crucible in general due to the difference
of material properties.
[0044] The formation and growth of a uniform cristobalite layer
either through pre-coating on the crucible or post addition of melt
modifier into the melt is started during stabilization mode prior
to crystal growth (i.e. after melting of poly silicon or during the
melting). In case of a post MM addition, the MM is introduced after
meltdown, so the formation speed of the cristobalite is slower than
otherwise pre-coated case. However, post addition of the MM can
yield lower air pocket (APK) losses because bubbles formed and
trapped at the crucible wall can be released to the surface prior
to the formation of the stable cristobalite layer. Because it
typically takes 3.about.7 hours from the start of the stabilization
step to the start of body growth, the cristobalite thickness is
estimated to be greater than about 2 mm with an appropriate amount
of MM added to increase the formation rate of the cristobalite. In
practice, a cristobalite layer of less than about 1.0 mm is
typically formed on the surface of the crucible (whether natural
sand or synthetic). Some special case, such as heavily doped
process for P++ or N++, yield a thicker cristobalite layer by
adding a large amount of MM. In such cases a cristobalite layer of
approximately 1.0 mm (+/-) is formed. These thicknesses differ from
the 2.0 mm because although the cristobalite layer is grown by hot
time as shown in FIG. 7, wet cristobalite layer is continuously
being dissolved to melt. Other embodiments include a cristobalite
layer of approximately 2.0 mm, greater than 1.5 mm, greater than
1.0 mm, greater than 0.75 mm, greater than 0.5 mm, or greater than
0.25 mm formed on the wettable surface of the crucible. In some
embodiments the cristobalite layer is less than 3.0 mm thick, less
than 2.0 mm thick, less than 1.25 mm thick, or less than 1.0 mm
thick. In some embodiments, the cristobalite layer falls within a
range defined by the minima and maxima above, such as between 0.25
mm and 1.25 mm. Generally, too thin a cristobalite layer may be
insufficient to provide the benefits described herein, while too
thick a cristobalite layer may be more likely to break and enter
the melt (potentially contributing to an LZD) during silicon
production.
[0045] Melt modifier addition described above forms a uniform
crystalline layer at the crucible wettable surface and this
crystalline structure has a strong resistance against the
thermomechanical stress induced by the irregular (transient) melt
flow produced during HMCZ. The formation of a thick and uniform
cristobalite can resist the stress and impact from the melt flow,
reducing crucible surface damage (i.e., resisting damage generating
quartz particles in the melt) which will increase the ZD
success.
[0046] As mentioned previously, a strong convective flow caused by
higher crucible rotation will reduce the variation of temperature
in the melt free surface and reduce the melt flow induced by a
horizontal magnet as seen in Case 6 in FIG. 5. In case 6, C/R is
1.6 RPM as compared to 0.6 RPM in cases 1-5 and 7. In other words,
the force produced by the magnetic field from the melt to the
crucible wall surface is reduced or blocked by the convective flow
related with higher crucible rotation. Thereby, the possibility of
quartz piece generation due to damage of crucible wall surface is
decreased, also improving the potential of ZD success.
[0047] A test condition was performed to understand ZD success and
on both synthetic lined and natural sand crucibles in a horizontal
magnet as a function of crucible rotation speed and the quantity of
melt modifier addition. The synthetic crucible showed high ZD
success ratio across a broad range of crucible rotation and MM. For
the natural sand lined crucible, a total of 96 trials with 19
different conditions were completed and the results are summarized
with a contour plot in FIG. 8. The data for FIG. 8 was from
approximately 200 mm body length to approximately OE of the
crystal. In FIG. 8, CR is the crucible rotation in RPM, a Success
%_3 of 0.0 is a ZD fail (LZD), and a Success %_3 of 1.0 is 100% ZD
success. The sign of the crucible rotation indicates direction of
rotation of the crucible. For some pulls, the crucible rotation
ramped between speeds during portions of the pull, an arbitrary
value of crucible rotation was selected for plotting, and the data
was collected from greater than 1000 mm body length to
approximately OE of the crystal. Results clearly show that higher
ZD success ratio is achieved with increasing the absolute value of
the speed of crucible rotation and quantity of melt modifier (in
this case, Ba based).
[0048] The use of increased speed of crucible rotation and the
creation of a cristobalite layer are shown to individually and in
combination result in improved ZD outcomes. At lower C/R, such as
between about 0 and about 2 RPM, addition of suitable MM (or use of
precoated crucible) is needed to increase the ZD success ratio. As
the C/R is increased to between about 2 RPM and about 5 RPM, the
amount of MM needed to gain an equivalent success rate decreases
and may even be zero MM. Within this range, additional gains in ZD
success ratio may be obtained through use of at least some MM. When
the C/R exceeds about 5 RPM there is generally no ZD concern from
the crucible and no MM is likely to be needed. However, at such
speeds, additional process condition will likely be required to
control the quality of the products, because it may have other
issues such as oxygen control due to melt flow velocity, melt level
control caused by centrifugal force, and the like.
[0049] Thus, some embodiments of the present disclosure use a MM
more than 1.7 grams per square meter of the wettable surface area
of crucible during an HMCZ process at any C/R rate to improve the
ZD success rate. In some embodiments, a MM between 1.7 and 2.0
grams/m.sup.2 of the wettable surface of the crucible is used. In
still other embodiments, a MM between 1.7 and 5.4 grams/m.sup.2 of
the wettable surface of the crucible is used. In still other
embodiments, a MM greater than 5.4 grams/m.sup.2 of the wettable
surface of the crucible may be used, but the large amount of MM
might cause LZD in multiple recharge processes. The quantities
above (e.g., 1.7 grams/m.sup.2) are based on BaCO3 as the MM.
Similar embodiments using BaO or SrCO3 include an amount of the
particular MM functionally equivalent to the amount of BaCO3.
[0050] Some embodiments use a MM between about 0 and 0.5 g/m.sup.2
and a crucible rotation greater than about 2.0 RPM. In some such
embodiments, the crucible is a natural sand crucible.
Alternatively, the crucible may be a synthetic crucible.
[0051] Embodiments of the methods described herein achieve superior
results compared to prior methods and systems. For example, the
methods described herein facilitate producing silicon with a higher
ZD success rate than some other methods.
[0052] When introducing elements of the present invention or the
embodiment(s) thereof, the articles "a", "an", "the" and "said" are
intended to mean that there are one or more of the elements. The
terms "comprising", "including" and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements.
[0053] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about,"
"approximately," and "substantially," is not to be limited to the
precise value specified. In at least some instances, the
approximating language may correspond to the precision of an
instrument for measuring the value. Here and throughout the
specification and claims, range limitations may be combined and/or
interchanged; such ranges are identified and include all the
sub-ranges contained therein unless context or language indicates
otherwise.
[0054] As various changes could be made in the above without
departing from the scope of the invention, it is intended that all
matter contained in the above description and shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
* * * * *